1. Field of the Invention
The invention relates to a digital transmission system comprising an equalizer for producing binary estimates from a sequence of sample values of a signal, to which estimates can be assigned binary values "zero" or "one" of issued bits (binary digits). Each sample value depends on the bit that can be assigned thereto and on n bits immediately preceding this bit, in that 2.sup.n states are assigned to each sample value and a probability function is assigned to the probability of the transition from one state to a following state and paths are formed by stringing together state transitions, an overall probability function being produced from all the probability functions of a path, and from all the paths leading to a state only the path having the minimum overall probability function is being considered.
2. Description of the Related Art
In digital transmission systems comprising dispersive transmission channels a signal coming from the transmitter and containing the information to be transmitted in the form of bits is distorted as a result of delay differences and phase shifts when passing through the transmission channel. These distortions cause the bits contained in the received signal to be influenced by preceding bits (intersymbol interference).
Dispersive transmission channels are found both in radio transmission systems and in cable transmission systems. Delay differences and phase shifts occur, for example, as a result of reflections and, in radio transmission systems, as a result of multipath propagation. Especially in radio transmission systems, in which send and receive stations are mobile, for example, in a car telephone system, these distortions additionally vary with time. Further distortions occur because the transmitted signals are superimposed by noise or other disturbing signals. In order to recover the original data contents of the signal it is necessary for the received signal to be equalized.
In this context, for example, from the prior art described in European Patent Application EP-A-0 294 116, it is known that the originally transmitted data sequence is reconstructed by implementation of the so-called Viterbi algorithm. In a receiver the sample values are produced by a sampling of the received signal, which values are applied to a Viterbi decoder. At least one sample value can be assigned to each transmitted bit. Due to the dependence of a sample value on n previously sent bits the Viterbi decoder performs, by means of likelihood computations, an estimation of what binary value a transmitted bit must have had.
In addition, for example, European Patent Application EP-A-0 152 947 discloses a Viterbi decoder. Viterbi decoders are used as channel decoders in transmission systems, a channel encoder being arranged at the send end and a channel decoder at the receive end. In such transmission systems a bit string to be transmitted is converted prior to transmission into an encoded data sequence by means of an encoding method. To this end, a convolutional code is used for encoding purposes in the channel encoder, according to which code a plurality of data elements of the encoded data sequence are generated by means of different logic operations of the bit to be encoded and of the m bits preceding the bit to be encoded. With this encoding it is advantageously possible that, in the form of the intentionally produced redundancy of the encoded data sequence, the channel decoder can recover the bit string originally forming the basis of the encoded data sequence in a data sequence to be decoded of which several data elements were received incorrectly.
The Viterbi method is thus suitable both for equalizing the signals that have passed through a dispersive transmission channel, and for decoding encoded data sequences. The Viterbi method as such is described, for example, in the article entitled "The Viterbi Algorithm", by G. David Forney Jr., Proceedings of the IEEE, Vol. 61, No. 3, March 1973, pp. 268-278.
For a so-called Soft Decision Decoding reliability information for the data sequence to be decoded will be necessary for the data elements to be decoded of an encoded data sequence. The reliability information is a probability function indicating with what probability the data elements to be decoded correspond with the transmitted data elements.
In the above article it is outlined how reliability information can be procured by means of a Viterbi decoder. The reliability information produced thereby is reliability information averaged over a plurality of successive bits in a bit string. The reliability information is hereby formed from the optimal and from the sub-optimal, for example limited number of the best solutions. This instruction on the formation does not seem ideal.